This invention relates to a tube clamping assembly used in a tube cutting apparatus. More particularly, it relates to an improved clamping overload assembly to prevent overly high forces on a clamp die set.
Tube cutting apparatus are well known in the prior art and typically include a cutting blade member and a die set having die jaws for clamping and holding a tube member while the cutting blade member cuts the tube. In some tube cutting apparatus a scarfing blade scores the tube to provide an initial cut for the cutting blade. These apparatuses are relatively complex since the tube is clamped and cut as it exits a mill, with the blades, the die set and the tube all moving in conjunction with one another at the same speed.
Various methods are used to ensure proper timing between the clamping of the tube by the die set, the scoring of the tube by the scarfing blade and the cutting of the tube by the cutting blade. In one known methods, a mechanical die jaw clamping cam is utilized with two slide members, each of which mount at least one die jaw of a die set, and each of which have cam followers in contact with the die jaw cam. Each slide member may mount a pair of axially spaced die jaws, and although the singular term die jaw may be used in this application, it should be understood that a pair of die jaws may actually be used. The die jaw cam is moved and forces the die jaws to engage and clamp the tube in timed actuation with the cutting blade and scarfing blade, if one is used, ensuring a clean cut. The timed sequence may be to clamp, move the scarfing blade to score the tube and then move the cutting blade to cut the tube. During scarfing and cutting the die jaw cam maintains a clamping force holding the die jaws in clamping engagement.
It is important that the die set firmly hold the tube to ensure a clean cut. As the blades cut the tube, large forces are placed on the die jaws in opposition to the clamping force holding them in clamping engagement with the tube. If the die jaws move away from clamping engagement, the tube will often be improperly cut. Thus, in the prior art, the die jaw cam places large clamping forces upon the die jaws to ensure that they maintain clamping engagement with the tube. An example of such a die jaw cam is shown in U.S. Pat. No. 4,108,029 issued to Alexander Borzym. In some systems, the die jaw cam may be forced downwardly by the same press which actuates the cutting blade. Thus, large forces may be placed on the die jaw cam.
An obstruction may be encountered in the path of a slide member or die jaw as it moves into clamping engagement with a tube. When this happens, the movement of the die jaw may be stopped. The die jaw cam will still attempt to move the die jaw and may damage components of the clamping system.
The types of obstructions which may typically be encountered in this environment are chips from previously cut tubes or other debris which might be in the area. Also, a tubing mill upstream of the clamping system may sometimes form tubing of a slightly larger diameter than expected. The cam is designed to clamp tubing of the excepted diameter. Thus, when a tubing section of overly large diameter is clamped by the clamping system an obstruction to the die jaw movement is encountered. The same is true if the tubing is somehow misaligned.
U.S. Pat. No. 4,294,147 issued to John Borzym, discloses an overload biasing assembly that allows a roller associated with one die jaw to slide relative to the die jaw, against an overload biasing force, and accommodate a die jaw cam in the event that an obstruction is encountered. In this device, a mechanical spring biases a cam follower into contact with the die jaw cam. An obstruction force, as could be expected as a die jaw cam attempts to move a die jaw through a path blocked by an obstruction, may be generated on the roller. If the obstruction force exceeds the overload biasing force, the roller moves to accommodate the die jaw cam without associated movement of the die jaw. Once this happens, the die jaw will no longer be moved towards the tube.
The forces normally encountered by the cam follower, even in the absence of an obstruction, are quite large. For this reason, the force on the cam follower in the prior art overload biasing assemblies may have exceeded the overload biasing force, even in the absence of an obstruction. The cam follower would then move to accommodate the die jaw cam, which is undesirable since the tube then may not be adequately clamped, and imperfect cuts may be experienced.
In an attempt to overcome this problem, the prior art used mechanical springs of increased spring force to provide a greater overload biasing force. These large forces could damage the cam follower and thus required correspondingly larger cam followers to withstand the constant stress from the spring, and also required other components to be made stronger and bulkier. To be practical, the prior art devices were eventually designed with numerous bulky linear springs. In one example there are more than ten such springs.
In addition, these linear springs are compressed as the cam follower moves to accommodate the die jaw cam. As this happened the spring force increases greatly and overly large forces are applied to the cam followers, resisting further movement. The overload biasing force provided by a linear spring may be undesirable since it may be preferable to have the cam follower move easily relative to the slide member and accommodate the die jaw cam once an obstruction force in excess of the overload biasing force is experienced.
Further, prior art overload assemblies applied an overload force to the die jaw which could be called the "scarfing blade" slide member. As the scarfing blade moves over the tubing to score and form an initial cut for the cutting blade, it moves over one or both of the two die jaws. Binding between the scarfing blade and the tubing is often encountered, and thus relatively large forces may push or pull on one of the die jaws in opposition to the force holding it in clamping engagement. This die jaw will be defined as a "scarfing blade" die jaw. It may not be practical to leave the tubing unclamped whenever this binding occurs, since it occurs frequently. For this reason it was necessary to have the overload biasing force on the "scarfing blade" slide member be high to compensate for this binding force from the scarfing blade.
Of course, it would be beneficial to eliminate this restriction on the design of the overload biasing force. It would be preferable that the overload biasing force be selected in response to a maximum force desired on the clamping system, rather than having the additional variable of accommodating any binding force created by the scarfing blade.